In the prior art, thermoplastic elastomers as typified by polyester elastomers, thermoplastic block copolymers and thermoplastic polyurethane have a relatively good resilience despite a low hardness and are widely used in the manufacturing of golf balls. However, these thermoplastic elastomers tend to lower rebound resilience, strength, impact resistance and low-temperature performance when they are tailored to a higher hardness.
On the other hand, ionomer resins in the form of copolymers of α-olefin with α,β-unsaturated carboxylic acids, neutralized with mono- to trivalent metal ions, are very tough and fully resistant to rupture even when undergoing substantial deformation at a high speed. Additionally, the ionomer resins are thermoplastic and easy to mold and work. On use as the cover stock for golf balls, this feature is very advantageous in improving the scuff resistance and durability against repetitive hits of golf balls. However, ionomer resins are short of flexibility and likely to give a hard feel upon impact.
An attempt was made to prepare golf balls from resin compositions in which thermoplastic elastomers are mixed with ionomer resins to compensate for the drawbacks of the respective components. For example, JP-A 56-83367 and JP-A 62-275480 disclose golf balls using resin compositions comprising polyester elastomers in admixture with ionomer resins. Since polyester block copolymers having flexibility and resilience are mixed with ionomer resins having toughness and a high modulus of resilience, the resulting mixtures make use of the advantages of the respective components and are golf ball-forming materials having improved rebound performance.
However, since polyester elastomers and ionomer resins are not so compatible when mixed together, the golf ball-forming resin compositions proposed in the above patent references have uneven morphology. They suffer from the problem that since the respective resins are readily oriented during injection molding, the molded parts are likely to delaminate and less durable against repetitive hits.
To improve the compatibility between ionomer resins and polyester elastomers, resin compositions comprising, in admixture, an ionomer resin, a polyester block copolymer and an epoxy-containing resin were proposed as golf ball-forming materials (see JP-A 9-176429 and JP-A 2002-356608). Since the compatibility between ionomer resin and polyester block copolymer is improved by the inclusion of an epoxy-containing resin, these resin compositions are golf ball-forming resin compositions having some delamination resistance, good flexibility and a high modulus of resilience. There is still left a room for further improvement in compatibility. The inclusion of an epoxy-containing resin can increase the melt viscosity of the resin composition. A high melt viscosity is advantageous for extrusion molding and blow molding, but inadequate for injection molding. The resin compositions are thus rather disadvantageous because injection molding is generally employed for golf balls.
It would be desirable to manufacture a golf ball featuring improved rebound, a soft feel upon impact, good durability and good moldability, using a golf ball-forming material which has advantages of both an ionomer resin and a thermoplastic elastomer, flexibility, rebound, toughness, flexural fatigue resistance, and tear strength, exhibits a sufficiently smooth flow in the melt to injection mold, and is free of delamination in molded form.